It is now almost two decades since the then Assistant Chief Geologist
of the US Geological Survey, Dr. G. Brent Dalrymple, described
polonium radiohalos as "a very tiny mystery."1 An expert
geochronologist, Dalrymple was being cross-examined in the Federal
District Court in Little Rock at the Arkansas "creation trial"
of December 1981.

Radiohalos (or radioactive halos) are minute spherical zones
of color or darkening surrounding tiny mineral crystals, all included
in larger grains or crystals of host minerals in certain rocks.
Alpha-particles produced by radioactive decay of U, Th, and their
decay products in the tiny mineral inclusions penetrate the surrounding
host minerals, damaging their crystal lattices and discoloring
them. The distances traveled by the a-particles are related to
their energies, and where the a-particles stop they do the most
damage, leaving spherical shells of intense discoloration. Because
the a-particles emitted by the radionuclides in the U and Th decay
chains have different energies, it is therefore possible to identify
which radionuclides were responsible for producing the radiohalos.
The minerals containing these radiohalos are usually studied in
thin sections, so the radiohalos are viewed in cross-sections
and thus exhibit ring structures.

Radiohalos produced by the 238U and 232Th
decay chains are thus easily explained. However, there are also
radiohalos found that only exhibit rings produced by the three
Po (polonium) radionuclides of the 238U decay chain
(Figure 1), and it is these Po radiohalos that are enigmatic.3
Examination of the tiny central mineral inclusions (or radiocenters)
in these Po radiohalos reveals that only the respective Po radionuclides
were present at the time the Po radiohalos formed, but their half-lives
are very short—218Po (3.1 minutes), 214Po
(164 micro-seconds) and 210Po (138 days). After 10
half-lives of decay the original quantities of radionuclides are
essentially exhausted, so these Po radiohalos rings would seem
to have formed very quickly, in approximately 31 minutes (218Po),
1.64 milli-seconds (214Po) and 1,380 days (210Po).

Now these Po radiohalos have been found primarily in the mineral
biotite, a mica—at 20 out of 22 reported localities.4
The rocks hosting these Po-radiohalo-bearing biotites at 17 of
the 20 localities are probably granites or granitic pegmatites.
According to conventional uniformitarian geology, such granitic
rocks formed over millions of years by cooling from hot magmas
intruded into the upper levels of the earth's crust.5
However, the radiohalos could only have formed after the biotites
had crystallized around the tiny Po-bearing inclusions and cooled.
Thus, assuming the Po was in the tiny inclusions when they first
crystallized, it can be concluded that the biotites, and therefore
the granites, had to crystallize and cool in less time than it
would have taken for the Po radiohalo rings to form6—1.64
milli-seconds for the 214Po radiohalo rings! If this
implies that these granitic rocks were instantly created, then
it is no wonder that the conventional geologist Dalrymple relegated
Po radiohalos to being "a very tiny mystery"!

Of course, such incredible implications have not gone unopposed.
Some observers have claimed that many Po radiohalos occur along
cracks and cleavage planes in biotites, and that fluids may therefore
have transported the Po into the biotites after the granitic rocks
had formed.7 In a relatively few instances this is
obviously true, but only 210Po radiohalos are found
along such cracks, which are sometimes also bordered by discoloration
with a width equivalent to the ring diameter of 210Po.
This implies that only the longer-lived 210Po was transported
in the fluids. Secondary 210Po radiohalos have also
been found in coalified wood associated with sandstone-hosted
U orebodies formed by groundwater flow.8 The Po was
concentrated from the U-transporting fluids into the Se-rich radiocenters
because of the geochemical affinity of Po with Se.

Critics also point to the apparent association between the occurrence
of Po radiohalos and concentrations of U—16 out of 20 Po-radiohalo-bearing
biotite localities have reported U in the rock or in an orebody.9
Thus it has been suggested that the fluids responsible for concentrating
the U may have also transported the Po and concentrated it in
the tiny radiocenters. 210Po is present in groundwaters,10
and in volcanic gases11 and fluids,12 and
has been reported in submarine hydrothermal vent fluids and chimney
deposits on the East Pacific Rise,13 where the 210Po
appears to have been transported over distances of up to several
kilometers in 20-30 days.

However, if Po radiohalos formed secondarily from fluid-transported
U-decay products, the expected amounts of the 218Po,
214Po and 210Po radiohalos would be directly
proportional to their different half-lives.14 Thus,
for example, there should be 67,000 210Po radiohalos
for each 218Po radiohalo. Yet in a Norwegian biotite
there are more than 1,000 210Po radiohalos, 90 218Po
radiohalos and only one 214Po radiohalo,15
but in other biotites the abundance ratios are 218Po>210Po>214
Po, and even 214Po>218Po or 210Po.16
Also, there may be as many as 20,000-30,000 218Po and
210Po radiohalos per cubic centimeter,17
or 5,000-10,000 218Po and 214Po radiohalos
per cubic centimeter.18 The seeming impossibility of
this secondary transport explanation is highlighted by the fact
that the 5 x 109 atoms of 218Po initially
needed to produce each very dark 218Po radiohalo had
to be concentrated in the tiny radiocenters in less than the 218Po's
three minute half-life.19 But experimentally-measured
diffusion rates are just too slow,20 and close to radiocenters
there is no large excess of a-recoil tracks left by decay of the
fluid-transported Po and Po-precursors.21

Another puzzle is that at five of the 20 Po-radiohalo-bearing
biotite localities the host granitic rocks intrude apparently
older rocks arguably produced during the Flood.22 If
these granitic rocks therefore also formed during the Flood, then
how were the Po radiohalos produced in them? On the other hand,
most of the Po radiohalos occur in Precambrian granitic rocks,
many of which might be related to the events of the Creation Week,
as might the one occurrence of Po radiohalos in a Precambrian
high-grade metamorphic rock.

Thus the Po radiohalos still remain "a very tiny mystery." There
can be no doubt, though, that they are significant as clues for
unraveling earth history within the Biblical framework, so further
research is warranted. If the Po radiohalos are indeed "fingerprints
of creation," 23 then they provide the means of identifying
Creation Week rocks. Similarly, they may indicate accelerated
radioactive decay in the past and/or suspension of normal geological
processes and process rates, including fluid transport, during
the Flood. Whatever these Po radiohalos are "telling us," we are
only going to find out by further "reading the rocks" to seek
a better understanding of the geological distribution and occurrences
of these Po radiohalos at both known and new localities. Such
research is now being pursued.24

Figure 1. Composite schematic drawing of (a) a
218Po halo, (b) a 238U halo, (c) a
214Po halo and (d) a 210Po halo with
radii proportional to the ranges of a-particles in air.
The nuclides responsible for the a-particles and their energies
are listed for the different halo rings [after Gentry2].